This is a common Chinese puzzle type. Number #3 is easiest. Look at the clue, when translated reads "72 hours". Okay, three days is the same as 72 hours. Take a look at the solution: the word is formed by three days (日【rì】). Got it now?

Now take a look at the first one. Translated "a mouth eats the cow's tail". Remove the word 尾, then remove the bottom of half of the word 牛, (the stem) and now add a 口 and you have the word 告.

These type of puzzles are often used to entertain large groups of Chinese people.